In my U.S. Pat. No. 4,612,093, issued Sept. 16, 1986, I described my discovery of a gold recovery process based on a novel "pregnant" electrolyte, comprising a halide solution impregnated with a nascent oxygen source. This process is practiced in an electrolytic cell, wherein the gold to be recovered is the anode. The cell cathode is an inert electroconductive substance isolated from the major portion of the electrolyte by a semipermeable membrane. This membrane is permeable to Na ions but not to the electrolysis-dissolved gold contained in the electrolyte. Thus, the gold does not deposit on the cathode but remains in solution. Portions of the electrolyte with the gold solution are removed from the cell and the gold is selectively precipitated by chemical reduction.
Any silver present in the original anode is precipitated as silver chloride. This insoluble compound precipitates in the vicinity of the anode and may be recovered as AgCl.
Any of the platinum group metals commonly used as gold alloying atoms remain in solution after the gold is chemically precipitated from the electrolyte. They can be recovered from the residual solution.
In U.S. Pat. No. 4,612,093 any silver present precipitates at the anode and may be filtered from the gold-containing pregnant electrolyte. In practice, the "pregnant" sodium chloride process is useful when handling small amounts of gold to be electrolyzed over short periods of time. I have found that when the concentration of gold in the electrolyte exceeds about 2-3 oz. per gallon of pregnant electrolyte, the rate of dissolution of the gold in the electrolyte decreases. In fact, even if the gold in the electrolyte is reduced by circulating the electrolyte from the cell and externally precipitating the gold, the rate of gold dissolution at the anode is markedly reduced. I have found that this reduction of gold solution is caused by an accumulation of a film of silver chloride at the surface of the anode. The formation of silver chloride at the surface of the anode physically separates the gold from the solution, slowing dissolution of the gold.
Also, during the use of alkali salts over extended periods, the alkali ions, together with the hydroxyl ions released at the cathode, form alkali hydroxides, such as NaOH in the case of the NaCl pregnant electrolyte, which slowly raise the pH of the electrolyte to such a degree that precipitation of hydroxides of the non-gold metals is encouraged. These extraneous hydroxides often contaminate the gold recovery from the electrolyte in the practice of the previous invention. Further, these precipitates foul the pores of the semi-permeable membrane and thus, over time, reduce the efficiency of the apparatus of that invention.